Low flux leakage magnet construction

ABSTRACT

A magnet construction is disclosed wherein the flux of a first, principal magnet is conserved by placing a second magnet adjacent the first magnet with the magnetic axes of the two magnets perpendicular to each other. The second magnet is constructed of a highly anisotropic material having low permeability perpendicular to its magnetic axis and preferably having a high coercive force and good magnetization retention. Preferably the second magnet surrounds the first magnet to minimize the flux leakage of the first magnet.

Unite States atent 1191 Neugebauer Oct. 23, 1973 [54] LQW FLUX LEAKAGEMAGNET 3,454,913 7/1969 lsraleson et a1 335/306 CONSTRUCTION OTHERPUBLICATIONS Inventor: Wendell Neugebauel" Ballston p W. Hofman,Strayfield Neutralizing, IEEE Transaction on Magnetics, v01. 2, No. 2,pp. 279, 280, June, 1970.

[73] Assignee: General Electric Company,

Owensbom, Primary Examiner-George Harris Att0rneyNathan J. Cornfeld eta1, [22] Filed: Apr. 3, 1972 [21] Appl. No.: 240,350 [57] ABSTRACT Amagnet construction is disclosed wherein the flux of 52 US. (:1 335/304,335/306, 310/254, 3 first, Principal magnet is conserved by Placing a 32 0nd magnet adjacent the first magnet with the mag- [51] Int. Cl. 1101f7/02 "@119 axes of the two magnets Perpendicular to each [58] Field ofSearch... 335/210, 302, 304, other- The second magnet is constructed ofa highly [56] References Cited UNITED STATES PATENTS 3,205,415 9/1965Seki et a]. 335/210 3,168,686 2/1965 King et a1. 335/306 anisotropicmaterial having low permeability perpendicular to its magnetic axis andpreferably having a high coercive force and good magnetizationretention. Preferably the second magnet surrounds the first magnet tominimize the flux leakage of the first magnet.

15 Claims, 8 Drawing Figures W N s 11/ v s 3 s I0 PATENTEU (1U 23 I973SHEET 1 BF 3 FIG.3.

PATENTEU OCT 2 3 I973 SHEET 2 CF 3 FIG ALNICO 5 l -8 H, Kilo osns rzosSHEET 3 [IF 3 PATENTED DU 2 3 I975 LOW FLUX LEAKAGE MAGNET CONSTRUCTIONBACKGROUND OF THE INVENTION Permanent magnets are normally characterizedas materials having magnetic flux producing properties generally alongan axis. However, in actual practice, some of the flux is generated indirections skewed from the main axis. This may be regarded as leakage orlost flux since the flux so generated contributes little, if any, to thedesired magnetic intensity. The extent of this loss varies with the typeof material as well as the geometric design of the magnet. In any eventthe usual remedy is to increase the total size of the magnet to achievethe desired magnetic intensity. This, however, has the un' desirableeffect of adding excessive weight to the magnetic system.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an improved magnet construction wherein the leakage flux isminimized.

It is another object of the invention to provide a magnet constructionhaving an improved magnetic intensity per unit weight ratio.

These and other objects of the invention will be apparent from thespecification and accompanying drawings.

In accordance with the invention the magnetic flux of a first magnet isconserved by placing a second permanent magnet adjacent the first magnetwith the magnetic axis of the second magnet perpendicular to the axis ofthe first magnet to prevent leakage of flux from the first magnet. Thesecond magnet comprises a highly anisotropic material having a lowmagnetic permeability in a direction perpendicular to its magnetic axis.Preferably, the second magnet comprises a high coercive force materialwith good magnetization retention properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of abar magnet constructed in accordance with the invention.

FIG. 2 is a cross-sectional view of a magnet for a klystron constructedin accordance with the invention.

FIG. 3 isa cross-sectional view of a magnet for a voltage tunablemagnetron constructed according to the invention.

FIG. 4 is a cross-sectional view of a modified version of the magnetconstruction of FIG.3.

FIG. 5 is a cross-sectional view of a loud speaker magnet constructedaccording to the invention.

FIG. 6 is a cross-sectional view of a portion of a motor having a rotormagnet constructionin accordance with the invention.

FIG. 7 is a cross-sectional view of a portion of a motor having a statormagnet construction in accordance with the invention.

FIG. 8 is a graph showing B-H curves for magnetic materials.

DESCRIPTION OF THE INVENTION prises a solid rod with the magnetic axisgenerally along the mechanical axis of the rod. Magnet 2 can be made ofany conventional magnet material such as, for example, steel or Alnico.

Magnet 10 is of generally conical configuration with a central boretherein conforming to the diameter of magnet 2 so that magnet 10 may beslipped over magnet 2. Magnet 10 is preferably conically shaped ortapered for a reason which will be described below. Magnet 10 can bepermanently attached to magnet 2, for example, by bonding the magnetstogether with generally magnetically transparent means such as epoxycement or the like. In accordance with the invention magnet 10 isconstructed of a highly aniostropic material having a low permeabilitynormal to its magnetic axis. Preferably, the material also shouldexhibit a high coercive force and good magnetic retention properties.The latter property is particularly desirable to prevent demagnetizationor alteration of the magnetic properties of magnet 10 by magnet 2.Materials which possess these properties and which have been found to beuseful in the practice of this invention include the rare earth-cobaltalloys described and claimed in US. Pat. Nos. 3,655,463 of Apr. 11,1972; 3,655,464 ofApr. 11, 1972; 3,684,593 of Aug. 15, 1972; and3,695,945 of Oct. 3, 1972; and all assigned to the assignee of thisinvention.

The use of the term rare earth-cobalt alloys is intended to include oneor more of the rare earth elements alloyed with cobalt. The term rareearth is intended to include the 15 elements of the lanthanide serieshaving atomic numbers 57-71 inclusive. The element yttrium (atomicnumber 39) is commonly included in this group of metals and is thereforeto be considered, in this specification, as also included in the termrare earth".

As shown by the arrows in FIG. l, magnet 10 is in accordance with theinvention, radially magnetized so that its direction of magnetization isgenerally perpendicular to the magnetic axis of magnet 2. Any skewing ofthe flux lines generated by magnet 2 are thus corrected or preventedfrom leaking out of the sides of magnet 2. This conservation or increaseof the available flux in magnet 2 enables one to reduce the overallsize, and thus the weight, of magnet 2 to a size commensurate with thedesired amount of flux without the previous necessity of additionallycompensating for flux losses by leakage of stray flux.

The magnetic material used in accordance with the invention for magnet10 must have low permeability in a direction normal to the magneticaxis, i.e. parallel to the magnetic axis of the main magnet 2 to preventshorting the field of the main magnet.

The term low permeability is intended to define a relative permeabilityof the cladding magnet material in a direction normal to its magneticaxis more like that of vacuum than that of ferromagnetic materials.

The cladding magnet material must also, in accordance with theinvention, have a high anisotropy, i.e., the material should be highlymagnetizable only along one axis. The degree of anisotropy ofa materialcan be found by determining the alignment factor which is the residualinduction (3,.) of the material divided by the saturation magnetization(4114,). If a theoretically perfect alignment factor was assigned avalue of l and an alignment factor representing completely randomalignment assigned a value of 0.5, the alignment factor representinghighly anisotropic material would have a value of at least 0.80 andpreferably 0.95 or higher.

As previously mentioned, the magnetic material used in constructingcladding magnet preferably should have a relatively high coercive forceto provide the required magnetic potential necessary for operation ofthe invention. Coercive force is a material property definedasthe fieldstrength (H,) at which the magnetic induction (B) becomes zero. Highcoercive force material, then, is defined in this specification asreferring to astrongly magnetizable material requiring a coercive forceof at least 2 Kilo oersteds, and preferably 4 Kilo oersteds, to reducethe magnetic induction to zero.

Referring to FIG. 8, B-H curves in the second quadrant of the hysteresisloops for various magnetic materials are plotted using the EMU system ofunits. It will be seen that the slopes of the curves vary, with thecobaltrare earth curves approaching a 45 slope, the theoretically idealcondition. The ratio of the value of H, to the residual induction B, themagnetic induction B corresponding to zero magnetizing force H is anindication of the magnetization retention properties of the mate rial.It can be seen that the value of H for the Alnico materials with respectto B, is much lower than that of the CoPt, ferrite and cobalt-rare earthmaterials and, in'

fact, such Alnico materials are unsuitable for use as cladding magnetsin accordance with the invention.

then, is defined in this specification as referring to a material havinga value of H at least 60%of B, (when expressed in theEMU system inOerstedsand gauss respectively) and preferably 80-90% or higher.

The term good magnetization retention properties,

Referring'now to FIG. 2, a specific application of the teachings of thisinvention is illustrated. A magnet comprising a single reversalpermanent magnet circuit for a linear beam tube is shown. Cylinders 13andl4 may be constructed of conventional magnetic material and aremagnetized axially and placed in opposition as shown. Cylinders 13 and'14 produce the flux that passes through pole pieces 15 and 16 and theopen space l9to be occupied by a linear beam tube such as I a klystronor traveling wave tube. Reversal pole piece 17 carries twice the flux aseither of the end pole pieces. J x a I "In accordance with theinvention, magnetic cylinders 7 l3'and 14 are jacketedby s qmem etential in cylinders 13 and l4-varies linearly with-dis-' tance from'the respective end pole pieces 15 and 16, the variation in thickness ofthe cladding 20 must be linear along the axis of magnets 13 and 14.Stated another way, inasmuch as the magnetic potential increases alongcylinders 13 .and 14, the magnetic counter potential necessary toprevent leakage of flux from cylinders 13 and 14 must beincreased byincreasing the size or thickness of cladding 20. The magnetic potentialofthe reversing pole 17 is constantand therefore the thickness of member20 is constant in this region. lron shelll8 carries no flux and ismerely added to keep external fields frompenetrating into the structure.

The thickness of member 20 at any point is dtermined by the magneticpotential on the principal magnet 13 or 14 as well as the coercive forceof the material used in constructing member 20. The shape andconfiguration of the cladding magnet or magnets can thus be determinedeither empirically or by calculations using the magnetic potential ofthe principal magnet and the magnetic properties of the cladding magnetmaterial. These parameters can, in turn, be determined by referring tothe 8-H curves for the particular materials used.

Thus, whilev i do not wish to be boundby any mathematical theoriesconcerning the magnetic forces, it is believed that the shape of member20 can be calculated by first determining the magnetic potential alongthe main magnets 13 or 14 by applying computational methods well knownto those skilled in the art and then applying the results to thefollowing formula:

' T u H,

where T is the thickness of. the cladding at any given point; U isthemagnetic potential (in gilberts in the EMU system of units) at thatpoint onthe surface ofthe main magnet; and H is the coercive force ofthe cladding material. v 7

Since the addition of the cladding material to the original magneticcircuit alters the flux pattern previously computed, it is essentialthat the computation be repeated with the cladding in place. The,newsolution, thus obtained will lead to a slightly different magneticpotential distribution U on the surface of the main magnets. Thecladding thickness is then slightly adjusted according to the aboveformulation. This process is repeated until a solution is obtainedthat'is self-consistent within the desired degree of accuracy Todemonstrate the method of computing the cladding thickness, the circuitof FIG. 2 will be analysed. Still referring to FIG. 2, the flux densityB isspecified to be axially directed with a complete reversal ofdirection in the center of the structure The magnetic field vector H istherefore also axially directed and has a magnitude equal to B in theEMU systemof units in the open space 19. Since no surface currentisflowing" at the boundary between the open space 19 and the cylindersl3 and 14, the field vectorHis the same. in the mate'rialof cylinders 13and 14 as inthe space l9 by. the law of continuity. ;;The.flumdensityofthe material ;of cylinders-l3 and l4'is now establishedgby referring tothe appropriate B-H characteristicof the material with v H be'ingknown.Designating B as the rnagriitudeof fluxdensity .inthe materialof thecylinders and,

and B the magnitude of flux density in the open space V l9, the rulethat'the divergence of the flux density vani sh implies that: Y I

I A,,,B,,, =.AB

where A, is the transverse area of the material comprisingcylinders l3and 14 measured in a plane perpendicular to the axis of these cylinders,and A is) the transverse" area of the open space 19 measured in the sameplane. This relation yields the. .value" of A because all theotherfactors are known or specified.

From this value of A the thickness-of the main flux, producing cylindersis determined.

The thickness of the cladding at any point X on; the surface of the mainflux producing cylinders is established by first determining themagnetic potential U along the outer surface of the cylinders 13 and 14and then referring to the appropriate B-H curve of the cladding materialto obtain the coercive force H which is the value of H when B is equalto zero. The values of U and H, are applied to formula (1) to determinethe thickness of the cladding at the point X. A similar procedure yieldsan equation for the cladding thickness along cylinder 14. Since the fluxpatterns were known beforehand, no repetition of the calculation isnecessary in this example, the given solution already beingself-consistent.

Although the method of computation outlined above yields a definitevalue of cladding thickness as a function of geometrical location, athickness different from the one computed may be used. In particular,changes in the thickness lead to changes in the flux density in the openspace to be occupied by a specific device, and thus afford a valuablemeans of adjusting the flux density for optimum device performance. Ingeneral, a lesser thickness of cladding yields a lower flux density andvice versa. The use of cladding thicknesses other than computed mayresult in some leakage flux which, however, is still far below theleakage flux normally associated with unclad magnetic circuits. Itshould also be noted here that non-tapered cladding, in some instances,may be used for manufacturing efficiencies.

Referring now to FIG. 3, another magnetic construction, in accordancewith the invention, is illustrated in a structural form most useful forcrossed-field devices such as magnetrons, voltage-tunable magnetrons(VTMs), and crossed-field amplifiers (CFAs).

In this construction the useful flux is produced by cylindrical bars 34and 35 magnetized in the same direction. This flux passes through thespace 36 to be occupied by the crossed-field device and completes thecircuit through iron shell 31.

In accordance with the invention, cladding magnets 32 and 33 comprisingradally magnetized cylinders sruuounding principal magnets 34 and 35.Again the thickness of magnets 32 and 33 is not uniform but is ratherdirectly proportional to the magnetic potential along the flux producingbars 34 and 35 or in the air gap 36. The cladding thickness, therefore,is zero at the very center of air gap .36 because the magneticpotential-is zero there.This proper adjustment of the thickness of thecladding magnet 32 and 33 results in no flux being carried by thesemagnets. Magnets 32 and 33 are constructed of magnetic materials havingthe properties previously described with respect to cladding magnet 10.FIG. 4. illustrates an alternate construction to FIG. 3 and useful inthe crossed-field devices previously described. In this embodimentmagnets 34', 35',

37 and 38 are each shaped as frustums of a cone. Magnets 34 and 37together comprise one main magnet while magnets 35' and 38 form theother main magnet.

Cladding magnets 32' and 33 have an outer cylindrical shape but are eachprovided with a tapered center bore conforming to the frustum shapes ofthe respective main magnets. An iron shell 31' surrounds the magnetconstruction to serve as a return path and to shield the magnets frompenetration by external fields.

To further illustrate the practice of the invention, a magnet wasconstructed as shown in FIG. 4. Frustums 34 and 35 were constructed ofAlnico 9 material with a base diameter of 1.25 inches, a diameter of0.875 inch at the smaller end, and 1.013 inches thickness. Magnets 37and 38 were constructed of Co Sm alloy having a base diameter of 1.390inches, a diameter at the smaller end of 1.25 inches, and a thickness of0.375 inch. Magnets 34, 35, 37 and 38 were all magnetized axially withthe North poles of magnets 35 and 38 facing the South poles of magnets34 and 37 as indicated by the arrows in FIG. 4.

Cladding magnet 32 was constructed of Co Sm alloy segments forming acylinder having an outer diameter of 1.65 inches and a length orthickness of 1.388 inches. Magnet 32' was magnetized normal to itscylindrical axis.

The segments forming magnet 32' were provided with an internal taper toprovide a conical bore to snugly receive frustum main magnets 34 and 37.The segments of magnet 32' were retained to magnets 34' and 37 usingEastman 910 cement.

The segments forming magnet 33 were similarly assembled about mainmagnets 35 and 38 and the clad magnet assemblies were mounted in ironshell 31' with the small ends of magnets 34 and 35 facing one another incoaxial alignment with a spacing therebetween of 0.550 inch.

Measurements of the flux density were made and found to be about 7,400gauss as compared to a theoretical value of about 8,000 gauss. While theflux densities of main magnets 34', 35', 37 and 38 were not actuallymeasured without cladding, standard magnets of this type, size, and gapare normally found to have flux densities below 5,000 gauss.

In FIG. 5 a loudspeaker magnet constructed in accordance with theinvention is illustrated. The main flux producing member is a radiallymagnetized disk 45 which is clad with axially magnetized shaped disks42, 43, and 44. The pole pieces 46 and 47 serve to concentrate the fluxto a density that is higher than that capable of being produced by thepermanent magnet material comprising disk 45. The large, generallycircular, block 41 comprises an iron return path for the flux from themain magnet 45.

The cladding or insulating magnets 42, 43, and 44 are, in accordancewith the invention, constructed of magnetic materials having theproperties previously described with respect to magnet 10. The axialmagnetization of magnets 42, 43, and 44 produces a magnetic counterpotentialperpendicular to that of radially magnetized disk 45. Thethickness of the cladding magnets 42, 43, and 44 is, again, proportionalto the magnetic potential on the surface of disk 45. Since this is notlinear along the disk, the thickness of magnets 42, 43, and 44 is not alinear function but is rather determined by the 8-H characteristics ofdisk 45 and the remainder of the magnetic circuit geometry.

It should also be noted here that in actual practice voice coil 48 musthave some means of external communication and thus magnet 44 must beeither modified or eliminated. This will result in a minor amount ofleakage flux. The amount of flux lost, however, with respect to theamount conserved is small and thus the object of the invention tominimize leakage flux is still carried out by magnets 42 and 43.

In FIG. 6, a motor is generally shown having a permanent magnet typerotor comprising radially magnetized spokes 52 having alternatedirections of magnetization. in accordance with the invention fluxleakage is minimized by the insertion of circumferentially magnetizedsections 54 between spokes 52 comprising materials having the magneticcharacteristics previously described with respect to magnet 10. Itshould be pointed "out again, however, that the principal magnets, inthis case spoke 52, can be constructed of any magnetic materialsdepending upon the flux density desired. The direction of magnetizationalternates for sections 54 to 7 provide the desired flux leakage buckingsystem in accordance with the invention.

Alternatively, as shown in FIG. 7, the stator may be constructed as apermanent magnet structure comprising radially magnetized spokes 62 cladin circumferentially magnetized sections 64. As discussed in previousembodiments each section 64 is not of uniform thickness but is rathertapered to provide a profile of strongest magnetic potential adjacentthe end of each spoke 62 facing the rotor. The amount of taper and thethickness is again determined as previously described.

For the correct choice of thickness of cladding magnets 64 the space 65between spokes 62 will be a magnetic field free region and may thereforebe filled with a highly permeable material. This space may,alternatively, be used by contouring the stator return shell 66 to fillthis region.

The cladding magnets used to provide magnetic potential in accordancewith the invention to inhibit leakage of flux from the main magnet aredimensioned to provide the correct amount of counter potential inaccordance with the potential along the main magnet as well as thecoercive force of the particular material used for the cladding magnet.The materials used for the cladding magnets must have the particularmagnetic properties previously described with respect to magnet 10. v

The cladding magnet may be shaped to the desired contour by pressing themagnetic material in particulate form to the desired shape followed bysintering of the shaped magnet. The magnetic alignment of the particlesbefore sintering and the magnetization of the sintered product arecarried out as described and claimed in the aforementioned pendingpatent applications.

The material can also be ground or machined to the desired shape. Whilethis would normally be done before magnetization because of thepractical problems experienced when processing magnetized materials, thedesired hig'h' anisotropy of the material may make it desirable tomagnetize the material before fabrication. When, as in the preferredembodiment, a magnetic material having high magnetization retentionproperties is used, processing can be done after magnetization withminimal risk of demagnetizing the cladding material.

The resultant cladding magnets are joined to the main magnet using, forexample, bonding means such as epoxy cement. Other bonding or mechanicalretention means can be used provided they do not interfere with themagnetization of either the mainmagnet or the cladding magnets. Incertain circumstances it may be desirable to remagnetize the main magnetin situ, i.e. after being clad when a material havin a low intrinsiccoercive force is used for the main magnet. Since, as previouslydescribed, the cladding magnet material is preferably characterized by ahigh magnetization retention, the choice of bonding methods or materialswill have little, if any, effect on its magnetization nor willremagnetization of the main magnet effect the cladding magnet. 7

Thus, the invention provides an improved magnet structure wherein theflux of a principal magnet is conserved by cladding the magnet with asecond magnet having its magnetic axis perpendicular to the magneticaxis of the first magnet and further characterized as preferably havinga high coercive force, with low permeability perpendicular to itsmagnetic axis, and highly anisotropic. Preferably, the aterial is alsocharacterized as highly resistant to alteration of its magneticproperties. The cladding magnet is preferably contoured as previouslydescribed to provide a perpendicular, bucking, magnetic forceproportionate to the potential along the principal magnet.

What is claimed is:

l. A permanent magnet construction having a minimum leakage fluxcomprising a first permanent magnet having a predetermined lineal extentand a magnetic axis therealong and a second permanent magnet forming acladding completely about said first magnet and coextensive therewith,said second magnet comprising a highly anisotropic material having itsmagnetic axis substantially perpendicular to the magnetic axis of saidfirst magnet and having low permeability in the direction normal to itsmagnetic axis to inhibit leakage of magnetic flux from said firstmagnet.

2. Themagnet construction of claim 1 wherein said second magnetcomprises a rare earth-cobalt alloy.

3. The magnet construction of claim 1 wherein said second magnetcomprises a material exhibiting good magnetization retention properties.

4. A magnet construction comprising a first magnet having apredetermined lineal extent and a magnetic axis therealong and a secondmagnet comprising highly anisotropic material having a low permeabilitynormal to its magnetic axis, said second magnet adjoining substantiallythe entire extent of said first magnet and substantially completelysurrounding the magnetic axis of said first magnet, the magnetic axis ofsaid second magnet being substantially perpendicular to the magneticaxis of said first magnet.

5. The magnet construction of claim 4 wherein the magnetic potential ofthe second magnet is not uniform along the magnetic axis of the firstmagnet.

6. The magnet construction of claim 5 wherein the magnetic potential ofthe second magnetic is strongest adjacent at least one end of themagnetic axis of the first magnet. v

7. The magnet construction of claim 5 wherein the thickness of thesecond magnet perpendicular to the magnetic axis of the first magnet istapered to provide a non-uniform magnetic potential along the magneticaxis of the first magnet.

8. The magnet construction of claim 5 wherein said second magnetcomprises a rare earth-cobalt alloy.

9. The magnet construction of claim 4 wherein said first magnetcomprises a plurality of magnets aligned along a common magnetic axis.

10. The magnet construction of claim 4 wherein said second magnetcomprises a plurality of magnets, each having its magnetic axis normalto the magnetic axis of said first magnet.

11. The magnet construction of claim 4 wherein said first magnetcomprises a pair of coaxially aligned hollow cylinders disposed inmutually axially spaced relation, and said second magnet is in the formof a double cone having a wide-thickness portion surrounding theadjacent ends of said cylinders, said wide-thickness portion being ofsubstantially uniform thickness over an axial extent thereofcorresponding to the spacing of said cylinders.

12. The magnet construction of claim 11 wherein said cylinders of saidfirst magnet are magnetized in opposite directions and said constructionfurther comprises a pair of radially extending reversal pole piecesdisposed within the space between said cylinders.

13. The magnet construction of claim 12 wherein the cylinders areadapted to surround a linear-beam electron discharge device.

14. The magnet construction of claim 4 wherein said first magnetcomprises a pair of spaced aligned solid bars of uniform diameter andhaving a common direcpied by a crossed-field electron discharge device.

1. A permanent magnet construction having a minimum leakage fluxcomprising a first permanent magnet having a predetermined lineal extentand a magnetic axis therealong and a second permanent magnet forming acladding completely about said first magnet and coextensive therewith,said second magnet comprising a highly anisotropic material having itsmagnetic axis substantially perpendicular to the magnetic axis of saidfirst magnet and having low permeability in the direction normal to itsmagnetic axis to inhibit leakage of magnetic flux from said firstmagnet.
 2. The magnet construction of claim 1 wherein said second magnetcomprises a rare earth-cobalt alloy.
 3. The magnet construction of claim1 wherein said second magnet comprises a material exhibiting goodmagnetization retention properties.
 4. A magnet construction comprisinga first magnet having a predetermined lineal extent and a magnetic axistherealong and a second magnet comprising highly anisotropic materialhaving a low permeability normal to its magnetic axis, said secondmagnet adjoining substantially the entire extent of said first magnetand substantially completely surrounding the magnetic axis of said firstmagnet, the magnetic axis of said second magnet being substantiallyperpendicular to the magnetic axis of said first magnet.
 5. The magnetconstruction of claim 4 wherein the magnetic potential of the secondmagnet is not uniform along the magnetic axis of the first magnet. 6.The magnet construction of claim 5 wherein the magnetic potential of thesecond magnetic is strongest adjacent at least one end of the magneticaxis of the first magnet.
 7. The magnet construction of claim 5 whereinthe thickness of the second magnet perpendicular to the magnetic axis ofthe first magnet is tapered to provide a non-uniform magnetic potentialalong the magnetic axis of the first magnet.
 8. The magnet constructionof claim 5 wherein said second magnet comprises a rare earth-cobaltalloy.
 9. The magnet construction of claim 4 wherein said first magnetcomprises a plurality of magnets aligned along a common magnetic axis.10. The magnet construction of claim 4 wherein said Second magnetcomprises a plurality of magnets, each having its magnetic axis normalto the magnetic axis of said first magnet.
 11. The magnet constructionof claim 4 wherein said first magnet comprises a pair of coaxiallyaligned hollow cylinders disposed in mutually axially spaced relation,and said second magnet is in the form of a double cone having awide-thickness portion surrounding the adjacent ends of said cylinders,said wide-thickness portion being of substantially uniform thicknessover an axial extent thereof corresponding to the spacing of saidcylinders.
 12. The magnet construction of claim 11 wherein saidcylinders of said first magnet are magnetized in opposite directions andsaid construction further comprises a radially extending reversal polepiece disposed within the space between said cylinders.
 13. The magnetconstruction of claim 12 wherein the cylinders are adapted to surround alinear-beam electron discharge device.
 14. The magnet construction ofclaim 4 wherein said first magnet comprises a pair of spaced alignedsolid bars of uniform diameter and having a common direction ofmagnetization, and said second magnet comprises a pair of cylindershaving a common axis and uniform internal diameters conforming with thediameters of said bars, the magnetic axes of said cylinders being inmutually opposed directions and normal to the magnetic axis of saidbars, and a magnetic shell encasing said first and said second magnetsto form a complete magnetic circuit.
 15. The magnet construction ofclaim 14 wherein the space between said bar magnets is adapted to beoccupied by a crossed-field electron discharge device.